Chlorination of Zirconium Compounds

V. N. Neustroeva, N. Ya. Negodina, G. M. Kuznetsova, V. M. Vityugin

(Presented by Prof. A. G. Stromberg, Doctor of Chemical Sciences)

Zirconium tetrachloride is obtained, depending on the technological scheme for processing zirconium concentrate into metal, by chlorinating three products: zirconium concentrate, zirconium dioxide, and zirconium carbide or carbonitride \[1, 2, 3, 7\].

Direct chlorination of zircon in a mixture with carbon has been introduced into production on a limited scale, owing to a number of reasons: the high working temperature of the process (on average 950°), the need for a constant supply of heat, and also difficulties in the condensation system because of the large amount of silicon chloride formed. Therefore, before chlorination the zircon concentrate is decomposed into zirconium compounds that are more readily chlorinated (zirconium carbide, carbonitride, zirconium dioxide).

The literature has considered the physicochemical principles and practice of chlorinating these zirconium compounds \[4, 5, 6, 7\]. Since zirconium oxychloride \((\mathrm{ZrOCl_2 \cdot 8H_2O})\) may also be obtained as an intermediate product in the processing of zirconium concentrate into metal, it was of interest to study the conditions for chlorinating this compound. This is all the more so because zirconium oxychloride is a compound that already contains chlorine and less oxygen. And since zirconium oxychloride is obtained at earlier stages of processing zirconium concentrate than zirconium dioxide \[1, 2, 4, 5\], its chlorination should be economically more advantageous than the chlorination of zirconium dioxide.

We carried out a study of the conditions for chlorinating zirconium oxychloride, zirconium dioxide, zircon, and calcium zirconate in order to identify possible advantages of obtaining zirconium tetrachloride from zirconium oxychloride.

Chlorination Procedure

Chlorination was carried out in an apparatus whose scheme is shown in Fig. 1.

Chlorine was obtained by the reaction between potassium permanganate and concentrated hydrochloric acid. It was dried in a system of absorbers with concentrated sulfuric acid. The gas-flow rate was regulated with a rheometer and in all experiments was kept constant, equal to 15 l/hr. Chlorination was carried out in a tubular electric furnace with a working-tube diameter of 25 mm. The starting materials, subjec-

those subjected to chlorination had a constant particle-size range of 5–3 mm. The charge of the starting material was 5 g and was loaded directly into the porcelain tube. The chlorination temperature was determined with a thermocouple placed in the reaction zone. The metal chlorides were trapped in a receiving flask kept in a thermostat at a temperature of 200–180°. The degree of chlorination was judged from the weight of the ignited residue after chlorination and from analyses for zirconium content in the residue after chlorination and in the starting materials.

Fig. 1. Diagram of the apparatus for chlorination
1—flask for obtaining chlorine; 2,5—empty Drechsel bottle; 3,4—Drechsel bottle with conc. H₂SO₄; 6—rheometer; 7—thermocouple; 8—porcelain tube; 9—galvanometer; 10—resistance furnace; 11—thermostat; 12—receiver.

Preparation of the Starting Material for Chlorination

Since under industrial conditions chlorination is, as a rule, carried out in shaft furnaces, briquetting of the starting materials was necessarily performed.

The starting compounds for chlorination were prepared by three methods:

1. Briquetting of pure zirconium compounds: zirconium dioxide, zirconium oxychloride, and calcium zirconate.

2. Briquetting of zirconium dioxide, zirconium oxychloride, zircon, and calcium zirconate with charcoal and coal-tar pitch by cold pressing.

3. Briquetting of the same zirconium compounds with charcoal and coal-tar pitch by hot pressing. The raw briquettes were partly subjected to direct chlorination and partly precoked at a temperature of 800–900°C. The resulting briquettes were crushed. For the experiments a 5–3 mm fraction was used. The composition of the charge for chlorination was: ZrO₂—60%, charcoal—25%, pitch—15%.

Results of the Investigation

The results of the investigation are presented in the table. Chlorination of zircon confirmed the data obtained by Aleksandrov [3]. He chlorinated zircon in a mixture with carbon at a temperature of 900°C for 6 h. The yield under these conditions was 93–95%. In our case, within 1 hour zircon was chlorinated by 30%, and in 6 hours by 92.60%.

In addition, we carried out experiments on the chlorination of zircon with additions of carbon and bleaching lime. The latter was added

Chlorination of zirconium compounds

Gas-flow rate 15 l/hour. Initial charge 5 g

Table

in an amount of 10% of the weight of zirconium. We assumed that bleaching powder could serve as an additional chlorinating agent, since its molecule is unstable and decomposes readily on heating.

Experiments showed that the addition of bleaching powder does not facilitate chlorination. The temperature at which chlorination begins rises, and the yield of chloride is 8% after 1 h of reaction. The residue after chlorination is a dense sintered mass, difficult to discharge from the furnace. Evidently, in this case the oxygen released during decomposition of the bleaching powder promotes the reverse reaction. Under the experimental conditions calcium chloride melts, coating the pieces of the starting substance. Pure calcium zirconate at a temperature of 900°C for 2 h is not chlorinated at all. With the addition of coal the degree of chlorination is likewise low—10—12% after 2 h of reaction.

Thus, as is evident from the experimental data, of the compounds studied, …

of zirconium compounds, zirconium dioxide and zirconium oxychloride in a mixture with carbon are chlorinated most readily.

The chlorination of zirconium oxychloride and dioxide was carried out at three different temperatures with an interval of 100°C. The duration of the experiment was chosen to be constant—1.2 hours—in order to make it easier to compare the results obtained.

Zirconium dioxide without the addition of carbon is still not chlorinated at a temperature of 900°C. According to Spitsyn [9], the initial temperature of interaction of zirconium oxide with chlorine is about 1000°. The addition of charcoal considerably lowers the chlorination temperature. In 2 hours at 600°C, zirconium dioxide with an addition of carbon is converted completely into the tetrachloride. Preliminary coking of the briquettes affects the initial temperature of interaction of zirconium dioxide with chlorine, lowering it from 460° to 400°C. As regards the yield of chloride, as is evident from the table, coking does not give any great advantage. At a temperature of 600°C the chlorination of both coked and uncoked briquettes is practically the same. The advantage of coking is manifested in charging the starting materials and in carrying out the chlorination process itself. Coked briquettes are stronger. During chlorination they do not crumble into powder. Uncoked briquettes are destroyed during chlorination, giving a dust-like mass which, when the rate of the chlorine stream is increased, is carried by it out of the reaction zone.

The chlorination of zirconium oxychloride in a mixture with carbon proceeds essentially in the same way as the chlorination of zirconium dioxide. At a temperature of 600°C in 2 hours the degree of conversion of zirconium oxychloride into chloride is 100%. Preliminary coking in this case also does not increase the yield of product, but likewise substantially strengthens the initial briquettes.

In contrast to zirconium dioxide, its oxychloride is chlorinated without the addition of carbon already at a temperature of 600°C. It is true that chlorination for 1 hour proceeds to 8–10%. A significant increase in the chlorination temperature, up to 900°, practically does not change the percentage conversion, nor does an increase in the chlorination time at a temperature of 600°C.

It should be noted that the conversion of zirconium oxychloride into the tetrachloride occurs more rapidly than that of zirconium dioxide. At a temperature of 600°C in 1 hour the degree of chlorination for the former was 97%, and for zirconium dioxide—88%. These data can be explained by taking into account the possibility of thermal decomposition of zirconium oxychloride. Takagi [8] showed that at a temperature of 400°C zirconium oxychloride decomposes rather readily according to the equation

\[ 2\mathrm{ZrOCl}_2 = \mathrm{ZrO}_2 + \mathrm{ZrCl}_4. \]

Hence the similarity in the chlorination conditions of zirconium oxychloride and zirconium dioxide becomes understandable. The observed differences, however, are associated with the onset of interaction (the initial temperatures of interaction, the degree of conversion of the starting material into chloride in 1 hour of reaction). In this respect, the fact is indicative that chlorination of zirconium oxychloride without the addition of carbon begins at a temperature of 400°C, while the yield of tetrachloride remains practically constant regardless of the temperature and duration of interaction.

Thus, in processing zirconium concentrate to metal, when zirconium oxychloride is obtained as an intermediate product, it is possible to chlorinate it instead of zirconium dioxide. This thereby eliminates the additional operations of obtaining zirconium dioxide from its oxychloride.

Conclusions

1. The chlorination of zirconium compounds—zirconium dioxide, zirconium chloroxide, zircon, and calcium zirconate—has been studied. It has been established that the conditions for chlorinating zirconium chloroxide are practically the same as those for zirconium dioxide.

2. It has been shown that, for chlorination under industrial conditions, the preparation of the materials to be chlorinated (briquetting, coking) is of great importance.

Literature

1. P. Forbath. Chem. Eng. No. 1, 128, 1958.

2. B. Lustman, F. Kerze. The Metallurgy of Zirconium, New York, 1955.

3. P. G. Aleksandrov. Ukrainian Chemical Journal, No. 2, 287, 1936.

4. G. L. Miller. Zirconium, Foreign Literature Publishing House, Moscow, 1955.

5. A. A. Cherepnev. Problems of Chlorination in the Field of Rare and Dispersed Elements, Metallurgizdat, Moscow, 1940.

6. I. S. Morozov, B. G. Korshunov. In: Chemistry of Rare Elements, vol. 2, 102, 1955.

7. G. A. Meerson, A. N. Zelikman. Metallurgy of Rare Metals, Metallurgizdat, Moscow, 1955.

8. Takagi. Chem. Abstrs. 49, No. 2, 765, 1955.

9. V. I. Spitsyn, O. M. Gvozdeva. Chlorination of Oxides and Natural Compounds, Proceedings of the Institute of Applied Mineralogy, Moscow, 1931.